1. Field of Invention
This invention pertains to a method and apparatus for detecting and measuring the flutter of free-standing web material in a web handling machine such as a paper web in a papermaking machine, and more particularly, by a method and device wherein the web flutter is determined from infrared light scattered from the web.
2. Brief Description of the Prior Art
Most paper is presently in the form of a continuous sheet of a preselected width in a papermaking machine. This sheet is usually called a paper web. During the manufacturing process, the paper web undergoes various treatments--including, for example, exposure to air jets or ventilation air for drying purposes--which cause complicated oscillatory motions of the web similar to the motions experienced by a flag. These motions, usually called flutter, can produce undesirable tensional forces on the web which can lead to edge cracks and/or web wrinkling which can cause papermachine breaks. Therefore, it is important in the paper industry to monitor web flutter to determine its characteristics, and sources.
Attempts have been in the past to develop methods and devices for monitoring and determining web flutter in a papermaking machine. However, these efforts yielded unsatisfactory results. For example, U.S. Pat. No. 4,501,642 to Wells discloses a method of paper tension control including a light source directing a light normal to the nominal plane of the web, and a light detector for detecting the light reflected from the web. However, this device cannot monitor paper web flutter with significant amplitude because as the web oscillates through its motions, the reflected light quickly surpasses the range of the light detector. Furthermore, the use of visible light is undesirable because, unless the measurement is taken in total darkness, the light sensor is also sensitive to ambient light, thereby confusing the measurements. Also, the sensor sensitivity can be quickly diminished due to the accumulation of fiber dust.
U.S. Pat. No. 4,637,727 to Ahola discloses a device for analyzing the reciprocating motion of a paper web by measuring the time required for a light beam to strike the web and return to the transmitter. This device needs very accurate elements for precise measurements, and in a manufacturing environment is susceptible to machine vibration and dust contamination. Furthermore, it also discloses a light sensor With limited angular range.
U.S. Pat. No. 3,703,097 uses an array of noncontacting proximity sensors to measure the flatness of sheet material. To remove substantially all internal compressive strains, the sheet is placed in tension, and the proximity sensors disposed at a base line are used to obtain a first set of measurements for the sheet on one surface. The sheet is reversed and a second set of measurements are obtained for the second surface of the sheet. The two sets of measurements are averaged to obtain an accurate transversal profile for the sheet.